Waveguide window



April 7, 1959 R. .1. BONDLEY 2,881,401

WAVEGUIDE WINDOW Filed'Jan. 12, 1954 IN V EN TOR.

Ralph J B ane leg Q/4 71 2/ H115 Attorney.

United States Patent O WAVEGUIDE WINDOW Ralph J. Bondley,

tric Company,

a corporation of New York My invention relates to dielectric windows foruse in hollow pipe type waveguides which are commonly used fortransmitting electromagnetic waves of ultra high frequency and moreparticularly, to a window which may be used to sectionalize such ahollow pipe type waveguide to permit operation of adjacent sectionsunder different pressure conditions.

It is customary to isolate the vacuum portion of electron tubes used forgenerating or amplifying energy at high power levels at microwavefrequencies from the remainder of the system by means of a hermeticallysaled dielectric window. To permit maximumpower to be transmittedthrough a waveguide it is necessary to keep the standing wave ratio inthe guide at a minimum; therefore, a window to be used in thetransistion section from the evacuated electron tube to a waveguidetransmission line must be designed to present a minimum discontinuity tothe transmission of the high frequency waves. Since the dielectricconstant of all suitable dielectric window materials is greater thanunity, it is necessary to alterthe geometry of the waveguide system atsuch a window to achieve a low standing wave ratio. This results in thecross-sectional area of the window being less than the cross-sectionalarea of the waveguide itself. Electrical failure in the system usuallymanifests itself at the window. On the air or high pressure side of thewindow, failure may occur by arc-over across the face of the dielectricwindow. On the vacuum or low pressure side, failure is usually due toerosion of the metal at the junction of the dielectric window and themetal frame to which the dielectric is hermetically sealed. Electricalfailure is particularly troublesome when a waveguide is operated at ahigh power level. For purposes of this description a high power level isconsidered to be that condition under which the electrical fieldgradient across the waveguide is very nearly that value which wouldresult in arc-over between two metal plates spaced the same distanceapart in air.

It is an object of my invention to provide a new and improved waveguidewindow, for use in a hollow pipe type waveguide system, that has acircular form in a plane perpendicular to the direction of propagationof high frequency energy in the waveguide system, that provides minimumattenuation to high frequency energy passing through the window and thatprovides a hermetical seal between two portions of a waveguide system.

Another object of my invention is to provide a window for use in awaveguide system operating at different pressures on either side of saidwindow that has a configuration which minimizes the tendency to arc-overon the Scotia, N .Y., assignor to General Elec- I high pressure side ofthe window and the tendency to erode the dielectric to metal seal on thelow pressure side of said window.

A further object of my invention is to provide a new and improvedstructure for sealing and supporting a dielectric window in a waveguidesystem.

A still further object of my invention is to provide a very strongdielectric to metal seal as a portion of the structure for sealing andsupporting a waveguide window in a hollow pipe waveguide system.

One of the featrues of my invention is the utilization of a circularwindow in the rectangular waveguide system. Several advantages resultfrom this shape window. Since the cross-sectional area of the windowmust be smaller than the waveguide itself, this particular symmetryresults in maximum distance between the regions of highest voltagestress. That is, in a rectangular waveguide of conventional design themaximum voltage stress will be across the narrow dimension of thewaveguide. This circular configuration decreases the likelihood ofarc-over across the face of the window and provides a window having across-sectional area less than the crosssectional area of the hollowpipe waveguide. To obtain a maximum creepage distance across the face ofthe window and thereby further reduce the likelihood of arc-over, and toreduce the accumulation of electrical charge on the low pressure side ofthe dielectric to metal seal, two dielectric flanges are provided whichextend beyond and on either side of the seal. The volume of thedielectric inside the window supporting frame or ring determines theresonant properties of the window. Therefore, the flanges or projectionsoutside the window area reduce the probability of electrical failurewithout altering the resonant frequency characteristics of the window.

It is well known that a rectangular metal waveguide of material such ascopper or brass may be utilized as a path for the transmission of highfrequency electromagnetic energy. Many types of high frequencyelectromagnetic waves may be transmitted through a hollow pipe typewaveguide although my invention is particularly adapted to systems fortransmitting waves of a mode where the electrical field occurs acrossthe narrow dimension of the guide such as the TE mode. It is recognizedthat this window will also operate and transmit energy occurring inother modes.

For a better understanding of my invention, reference may be made to thefollowing description taken in connection with the accompanying drawingsand its scope will be pointed out in the appended claims. Figure 1 is anend view of a wave guide window in accordance with my invention. Figure2 illustrates, by way of a crosssectional view along section 2-2 of Fig.1, one manner in which a window may be sealed into a waveguide structurein accordance with my invention.

Referring to Figures 1 and 2 of the drawing in which the same parts havelike reference numbers, there is illustrated a preferred structure forachieving the objects of my invention. Figure 2 illustrates in detailone manner of assembling a window in a hollow waveguide. There is showndisk 1 and ring 2 both of which may be of any suitable dielectricmaterial. In this case I used highfired alumina. Sandwiched between disk1 and ring 2 is an expansion member 3 made of pure nickel sheet. Theexpansion member 3 is bonded between disk 1 and ring 2 by means of ahigh melting-point solder. Such a solder may be formed by painting theparts with titanium hydride, applying a sheet of silver eutectic alloybetween the parts, and heating to a temperature of approximately 825-850 C. This produces a strong and hermetically tight seal between ring2, nickel member 3 and disk 1. This method of soldering is more fullydescribed and claimed in Kelley Patent No. 2,570,248 dated October 9,1951 and assigned to the assignee of this application. The nickelexpansion member is chosen because it is very soft and flexible andtherefore makes an ideal member to absorb the different rates of thermalexpansion between waveguide -flange 7 and the dielectric window disk 1and ring 2. A strong joint is formed by placing the expansion member 3between alumina disk -1 and ring --2 since ring 2 reduces the tendencyof a thin sheet of metal, such as nickel member 3, to tear away from aflat surface.

Expansion member 3 is welded to support ring 4 and support ring 4 iselectrically welded to support ring 5 by weld 6. Support ring 5 isconnected to waveguide flange 7 in any suitable fashion, in this case byelectric brazing a ring 8 over support ring 5, and thereby completes thestructure for hermetically sealing the waveguide window to the waveguideflange. .Waveguide flange '7 is designed to be connected to a similarflange by means of bolts which pass through holes 9. Support rings 4 and5 are, in this case, constructed of stainless steel having a coeflicientof thermal expansion approximating that of the copper or copper alloywaveguide flange. The differences in thermal expansion between thesupporting structure and the dielectric window are absorbed byflexibleexpansion member 3.

Disk 1 is cast and machined with flange 10 which has surfaces ll-a'nd 12forming a pair of faces against which faces 13 and 14, respectively, ofalumina ring 2 may rest, Face 12 is incontact with nickel expansionmembe! '3 over that portion'opposing surface 14. Face 15 isin contactwith the'expansion fold of member 3 only and does not contacttheremainder of member 3 or the support ring 4. There is thereby formeda long path around flange 10 which tends to limit the possibility ofare-over across the face of the disk 1. Alumina ring 2 is incontact withmember 3 over the area of face 14 only. Face 16 tapers away from thenickel member 3 and support ring 4 and makes no contact therewith. Face17 of ring-2 forms an effective protective shield for thebond betweenface 14, nickel member 3 and face 12. This shield protects and preventsdirect bombardment of the joint by the electrodeless discharge withinthe waveguide. Furthermore, the "long path from surface 17, alongsurface. 16, and finally to thejoint between disk 1 and ring 2 tends tolimit the build-up of electrostatic charge and thus erosion of thejoint.

Itisnotedthat the resonant frequency of the window 1-is determined bythe amount of dielectric material inside of surface 11. Therefore, theflanges formed by flange 10 of disk 1 and by alumina ring 2 do notafiect the resonant transmission characteristics of the window. Sincethewindow must be hermetically sealed and in addition must also withstandatmospheric pressure, the circular construction results in a symmetricalstrain pattern and hence a considerably greater freedom from cracking.

In this particular embodiment of my invention I have constructed awindow for a waveguide; the narrow dimension of which is approximately3% inches while the window flange diameter is approximately 4 inches.The relatively large'diameter of the window plus the extended portionsof the dielectric in the form of flange 10=a'n'd ring 2 results in avery long creepage distance such that little or no failure has beenexperienced by arcing-over across the face on the high pressure side ofthis window or by erosion on the low pressureside of thiswinddw.

It may be readily appreciated from the foregoing description that thereare numerous variations possible in the assembly of a window inaccordance with the objects of my invention. However, it may be seenthat the utilization of a welded joint 6 between rings 4 and 5 as wellas a separate dielectric to metal seal permits each bond to be of thetype to achieve an ideal supporting junction or an optimum metal todielectric seal respectively and results in an improved window forhermetically sealing a section of a waveguide system. Also theprojecting portions or flanges of dielectric protect the window andresult in a window that is not subject to break down due to arc-overacross the face on the high pressure side or erosion of the dielectricto metal seal on the low pressure side.

While I have shown and described my invention as applied to oneparticular embodiment thereof, it will be obvious to those skilled inthe art that changes and modifications may be made without departingfrom my invention, and therefore .I intend in the appended claims tocover all such changes and modifications as fall within the true spiritand scope of my invention.

What I claim as new and desire 'to secure by Letters Patent of theUnited States is:

l. A Window for use in high power microwave apparatus comprising arectangular waveguide, a metal ring, a substantiallyflat circular memberof dielectric material hermetically sealed to said metal .ring at acircular region spaced inwardly from the edge of said circular member,said circular region having a larger diameter than ..the transversedimension of said guide.

2. A window for use in high power microwave apparatus comprising a metalring, a dielectricmaterial herme tically sealed to said metal ring withportions of said dielectric material extending beyond this seal, on bothsides of and in planes parallel to and spacedtfrom opposite. sides-ofthe plane of saidlmetalring.

3. A window for use in high powerfmicrowave apparatus comprising ahollow waveguide, a metal ring-connected to said waveguide, a dielectricmaterial hermetically sealed to said metal ring with portions of saiddielec tric material extending beyondthe seal, on both sides of and inplanes parallel to the plane of said metal ring, the extended dielectricportions being separate from said ring.

4. A window structure for transmitting high frequency, high energy powercomprising a dielectric disk, a dielectric ring and an annular metalsupporting ring hermetically sealed to said disk at a circular regionspaced inwardly from the outer edge of said disk andsaid dielectric'ringbeing sealed to said annular metal ring at an area opposite said.region.

5. A window structure for transmitting high frequency, highenergy powercomprising a dielectric disk, at dielectric ring and a metal supportingring hermetically sealed between'said disk and dielectric ring, portionsof said disk and said'dielectric ring extending beyond the sealin planesparallel to the plane'of the metal supporting ring.

v6. .A window structure for transmitting high frequency, high energypower comprising a dielectric disk, a dielectric ring and a metalsupporting ring hermetically sealed betweensaid disk and dielectricring, portions of said dielectric disk-and said dielectric ringextending beyond the seal, separate from and in'planes parallel to theplane of the metal supporting ring.

7. A window structure for transmitting high frequency, high energy powercomprising a waveguide, a dielectric disk, a dielectriczrin'g and ametal supporting ring her metically sealed between the disk anddielectric ring, portions of said dielectric disk'and dielectricringiextending beyond the seal, separate fromand in'planes parallel tothe planes of the'metal supporting ring, and means for hermeticallysealing said metal supportingring in-said waveguidestructure transverseto the direction of propagation of highfrequencyenergy so that thewin'dowstrueture presents a gas tight separation between two portions ofsaid waveguide.

8. A window assembly for use in a waveguide system which will transmithigh frequency energy with minimum attenuation and which acts as a gastight seal between separate portions of said system, comprising awaveguide with a flange portion, a solid disk of dielectric material, anannular dielectric ring, a flexible metal expansion ring sealed betweenthe disk and dielectric ring and welded to a first supporting ring, anda second supporting ring Welded to said first supporting ring andsecured to said waveguide flange portion, portions of said disk anddielectric ring extending beyond the seal, separate from and in planesparallel to the plane of said supporting rings.

References Cited in the file of this patent UNITED STATES PATENTSFOREIGN PATENTS Great Britain Mar. 29, 1950

